1. Field of the Invention
The present invention relates to a printed wiring board for transmitting a digital signal at high speed.
2. Related Background Art
In the printed wiring board, a DC resistance component of a wiring conductor attenuates a signal by nature, thereby adversely affecting transmitted waveform quality. Accordingly, the longer a transmission signal line, the larger the attenuation, which results in reduction in signal integrity. In recent years, it has been found that in the case of a signal of a frequency band of 1 GHz or more, the influence of a skin effect or a dielectric loss is extremely large, and signal attenuation is much greater. Thus, in the signal transmission of a gigahertz order in the printed wiring board, the signal attenuation is one of the problems to be solved to secure signal integrity.
As one of indexes to evaluate signal attenuation, a transmission characteristic S21 parameter is known. The transmission characteristic S21 parameter is obtained by constructing a network for a transmission line as illustrated in FIG. 13A, and digitizing a rate (transmittance) of a signal transmitted from an input port (port 1) to an output port (port 2) of the network in this case. A digitized rate (transmittance) of a signal transmitted from an input port (port 2) to an output port (port 1) of the network is a transmission characteristic S12 parameter.
A relation between a frequency and attenuation of a signal to be transmitted will be described by using the transmission characteristic S21. FIG. 13B illustrates an example of general transmission characteristic S21 of a signal transmitted through a signal line provided in a printed wiring board. In FIG. 13B, the ordinate indicates transmission characteristic S21 (dB) of an S parameter, and the abscissa indicates a frequency (Hz) of a signal to be transmitted. In a frequency band of 1 GHz or less, attenuation caused by DC resistance of a wiring conductor is predominant, and attenuation gradually increases as the frequency becomes higher. This phenomenon is due to a skin effect or a dielectric loss of a high-frequency signal. Especially, in a frequency band of 1 GHz or more, because a loss caused by a skin effect or a dielectric loss is larger than a DC loss, the attenuation drastically increases. Further, large attenuation is observed at specific frequencies, which is due to resonance between inductance and capacitance components of the wiring conductor.
Causes of a reduction in signal integrity include not only signal attenuation but also impedance mismatching of a line. In other words, when mismatching occurs in impedance of the line, the transmitted signal is repeatedly reflected at the mismatching point to greatly reduce the signal integrity. Impedance mismatching greatly fluctuates depending on changes not only in connection point of the lines but also in width of the line, an interval with another line, and a dielectric constant around the line. An ordinary printed wiring board is designed such that, for example, the impedance characteristics are unified at 50Ω in a single ended line and at 100Ω in differential lines.
An impedance (Zo) of a line of a microstrip line structure can be calculated by the following (Equation 1):Zo=60/√(0.475×εr+0.67)×ln(4×h/(0.67×(0.8×W+t)))  (Equation 1)
In Equation 1, εr is a dielectric constant of a dielectric layer of the printed wiring board which is a lower layer of the wiring layer, h is a thickness of an insulating layer from a GND layer to the wiring layer, W is a width of the line, and t is a thickness of the line.
When the line is a differential signal line, a differential impedance (Zdiff) can be calculated by the following (Equation 2):Zdiff≈2×Zo(1−0.48×exp(−0.96×S/h))  (Equation 2)
In Equation2, h is a thickness from the GND layer to the wiring layer, and S is a spacing between two lines constituting a differential line.
Further, an impedance (Zo) of a stripline structure can be calculated by the following (Equation 3):Zo=60/√(εr)×ln(4×h/(0.67×(0.8×W+t)))  (Equation 3)
In Equation 3, εr is a dielectric constant of a dielectric layer of a printed wiring board which is a lower layer of the wiring layer, W is a width of the line, and t is a thickness of the line.
When the line is a differential signal line, a differential impedance (Zdiff) can be calculated by the following (Equation 4):Zdiff≈2×Zo(1−0.374×exp(−2.9×S/h))  (Equation 4)
In Equation 4, h is a spacing between two GND layers sandwiching the line, and S is a spacing between two lines constituting the differential line.
Japanese Patent Application Laid-Open No. 2006-173239 proposes measures against impedance mismatching caused by a change in line width. This patent document describes a structure when a line of a printed wiring board is connected to a connector. Because the land size of the connector is set larger than the line width, the line width of a portion near the land is set large to match the land size of the connector. In this case, impedance mismatching caused by the enlarged line width is suppressed by setting thick a lower dielectric layer of the portion with the enlarged line width.
Japanese Patent Application Laid-Open No. 2005-340506 describes a method of correcting impedance mismatching by changing a line width. This patent document describes a differential signal line which includes a first line and a second line for interconnecting driver and receiver elements mounted on a printed wiring board. The first line and the second line are connected to each of electrode terminals of the driver and receiver elements. In this case, the spacing between the electrode terminals is set larger than widths of the first line and the second line. Accordingly, the first line and the second line are connected to each of the electrode terminals by gradually increasing, near the electrode terminals, the spacing between the first line and the second line provided in parallel. Impedance mismatching caused by the enlarged spacing between the first line and the second line is suppressed by increasing the widths of the first line and the second line as the spacing between the first line and the second line increases.
As measures to suppress a reduction in signal integrity caused by signal attenuation which occurs due to a DC resistance component of a wiring conductor, enlargement of line width has hitherto been known. By enlarging the line width, a transmission sectional area of a signal is enlarged to enable reduction of the DC resistance component.
However, the enlarged line width reduces impedance characteristics of the line. The reduced impedance characteristics cause impedance mismatching, thereby reducing the signal integrity.
The impedance characteristics are enhanced by widening the spacing between lines. Accordingly, when a line width is set large, impedance characteristics can be set to a predetermined value by widening the spacing between the lines. However, when the spacing between lines is widened, a wiring area on a printed wiring board is increased, and terminal widths of a semiconductor package and a connector to which the lines are connected need to be set large. This has been a big obstacle to size reduction of printed wiring boards and semiconductor packages in recent years.